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Factors Controlling the Preservation of Porphyrins and Formation of Metalloporphyrins with Perspectives From Ancient Source Rocks and Modern Analogs


Cycloalkanoporphyrins are the geologically stable products of chlorophylls derivatives (CDs). They are useful as biomarkers, for source rock-oil correlation and for compound-specific isotope analyses. Porphyrins can also be useful for deciphering the redox history of the sedimentary environment from the distribution of metalloporphyrins. In ancient rocks and oils, porphryrins typically occur as complexes with divalent metals (VO, Ni, Cu, Fe, Zn and others). In recent environments CDs are present principally as chlorins and retain a high degree of functional similarity to their precursors. Here I will consider examples from the recent Peru Margin and the Cretaceous of Demerara Rise that illustrate some of complex factors that control the distribution of CDs in sediments. Three bicycloalkanoporphryins are present as free bases and as complexes with Zn, Ni, Cu and VO in Cenomanian-Turonian sediments of Demerara Rise. The stratigraphic distribution of porphyrins is controlled, foremost, by metal availability rather than sedimentary Eh/pH conditions or post depositional maturity. Titration of the local water column metal reservoir with sulfide and organic matter during Oceanic Anoxic Event 2 resulted in a high concentration of FB BiCAPs and a low concentration of metalloporphyrins. Conversely, high metal concentrations occur in sediments above and below the OAE, where porphyrin concentration are also highest. The lowest concentrations occurred during the heart of OAE 2, where enhanced preservation may be expected; we suggest this was the result of decreased preservation of tetrapyrroles in the absence of available metals. The high reactivity of Zn2+ with sulfide limited the formation of Zn complexes to the non-sulfidic regions in the water column or at the sediment/water interface. The formation of VO porphyrins occurred exclusively within the sediments from available FBs or through transmetallation with other metallo-porphyrins (Zn, Ni, Cu). Surface sediments from the Peru Margin oxygen minimum zone are subject to lateral and downslope transport by bottom currents that decrease organic matter quality despite low oxygen conditions. Degradation of CDs (chlorophyllone, pheophytin and pyropheophytin) demonstrate that the most significant degradation occurred between 150 and 400 m water depth, in the heart of the oxygen minimum zone. The continued action of anaerobic bacteria during extended residence times at and above the sediment water interface.